Conventional surgical procedures are carried out utilizing a sequence of surgical instruments or tools. At the outset of a given procedure, sharp mechanical devices, such as the scalpel, are employed to part the skin layers so as to provide external access to the body cavity. Bleeding during such initial stages may be controlled through the use of ties, clamps, blotting procedures, and the like. As the body cavity is accessed, tissue not only is cut but is manipulated to the extent that mechanically sharp instruments often are supplanted by blunt counterparts. These blunt counterparts electrically perform cutting and blood coaggulating functions on demand through closure of a switch. Such electrosurgical technology has been available to surgeons for decades. For instance, a monopolar electrosurgical device was developed over sixty years ago by William T. Bovie. This early device described, for example, in U.S. Pat. No. 1,813,902, issued Jul. 14, 1931, entitled "Electrosurgical Apparatus" has met with acceptance over the years within the surgical community to the extent that current versions are referred to as the "Bovie". Such devices typically consist of a handle having a first or "active" electrode extending from one end. The other end of the handle is electrically coupled to an electrosurgical generator which provides a high frequency electric current in either an a.c. cutting mode or pulsed coaggulating mode. A remote control switch is attached to the generator and commonly is present as a foot switch located in proximity to the operating theater. During an operation, a second or "return" electrode, having a much larger surface area than the active electrode, will be positioned in contact with the skin of the patient. To remove tissue, the surgeon brings the active electrode into proximity with the tissue to be cut or coaggulated. This is a starting condition where the instrument has not touched tissue. At this point in time, the electrical switch is actuated whereupon the active electrode is brought into close proximity with the tissue to be cut. Electrical current then arcs from the active electrode and flows through tissue to the larger return electrode. In a cutting mode, the electrical arcing and corresponding current flow results in a highly intense but localized heating which causes cell destruction and tissue severance. Following a short cutting routine, the instrument again is elevated in still air away from the tissue for two or three seconds. In general, the device can be switched to a pulsed, higher voltage input to perform in a coaggulating mode.
Another common modality for electrosurgery is referred to as bipolar electrosurgery. With this approach, no large return electrode is in contact with the patient. Instead, each instrument is made having first and second electrodes arranged in close mutual proximity. The device is utilized with a dedicated bipolar cable which is inserted in appropriate bipolar outlets of an electrosurgical generator, a device found essentially in all major health care facilities. When switch activated, the bipolar device provides an electrical current which arcs from the end of a first electrode to the second. Tissue disposed between the electrodes is cut and blood may be coaggulated. In general, surgeons are trained in the use of both bipolar and monopolar modalities, however, particularly in conjunction with endoscopic applications, bipolar devices are becoming more accepted in view of safety considerations. In the latter regard, the bipolar approach overcomes certain of the more undesirable characteristics of monopolar instruments in that excessive necrosis is reduced and current is not passed extensively through the body of the patient. Since current arcs between adjacent electrodes, blood vessels readily are cauterized. Bipolar devices, however, generally exhibit a lesser quality cutting ability, and it is often difficult to accurately locate the arc between the two electrodes with respect to tissue under resection.
Typically, the ubiquitous electrosurgical generators exhibit outputs with frequencies ranging from about 350 KHz to 1 Mz. Such higher RF frequencies serve to avoid tissue stimulation which would otherwise occur at lower frequencies.
Investigators also have considered the implementation of resistive heating to carry out coagulation and cutting in surgery. Early devices employed a very fine wire formed as a loop or extending linearly between spaced mounting points. Formed, for example, of platimun, the thinness or small diameter of the heated wire was required in order to gain a high enough resistance to develop correspondingly high enough temperature levels in conjunction with practical current levels. The requisite thinness of the wire resulted in marginal strength or rigidity, thus restricting applications of such instruments to to spot coaggulation or very limited cutting procedures. Surgical blades have been developed with mechanically sharp edges and side mounted electrical heating elements. With these instruments, cutting is at the mechanically sharp facet of the blade, and the coaggulation or hemostasis is intended to develop as a result of contact of the sides of the blade with the cut tissue. This, unfortunately, represents and attempt to stop bleeding after cutting, as opposed to a more desirable procedure for simultaneous cutting with coaggulation. Of course, the possible damage resulting from use of a sharp instrument within the body cavity is inherent with these instruments. Another problem associated with these earlier thermally based surgical devices is that the wire or blade will rapidly cool upon contact with tissue. As the blade cools, it becomes less and less effective for providing hemostasis. Additionally, as the blade cools below a temperature threshold, tissue will tend to stick to it, resulting in an obstruction of the cutting edge. If additional power is supplied to accommodate for the cooling effect, overheating may occur in some regions of the blade. Such overheating may be accompanied by unwanted tissue burning or blade destruction.
More recently, resistively heated thermal devices have been introduced which may employ blunt cutting portions providing both cutting and coaggulation. Such blunt cutting portions particularly are desirable for use with endoscopic applications. These devices utilize a thermal cutting portion emulating surgical blades and other implements which exhibits a self-regulating temperature characteristic. Self-regulation (also known as auto-regulation) involves maintaining the cutting surface of the surgical device within an elevated preselected temperature range. An approach for attaining self-regulation has been to employ a ferromagnetic material in constructing the end or heating element of the surgical instrument. When RF current is passed through such ferromagnetic material, current density tends to concentrate near its outer surface. This current density attenuates exponentially as distance into the material from the surface increases, a phenomenon known as the "skin effect".
The depth of the skin effect, i.e. the distance of penetrating current density into the ferromagnetic material, is defined as the depth at which current is reduced to approximately 37% of its surface value. This depth may be presented mathmatically as follows: ##EQU1## where skin depth is measured in centimeters, .rho. is electrical resistivity in ohm-centimeters, .mu. is electrical relative magnetic permeability for the ferrogmagnetic material, C is a constant, e.g. 5.times.10.sup.3, and f is frequency of the applied alternating electrical potential.
In ferrogmagnetic materials, such as iron, nickel, cobalt, and respective alloys, adjacent atoms and molecules couple their magnetic moments together in rigid parallelism (an interaction known as exchange coupling) in spite of the randomizing tendency of the thermal motion of atoms. If the temperature of such material is raised above a "Curie" temperature, specific for each ferromagnetic material, the noted exchange coupling suddenly disappears. As a result, these materials exhibit large changes in relative permeability as the temperature of the ferromagnetic material transitions through Curie temperature. As seen from equation (1), since the relative permeability is known to change in response to the temperature of the material, the associated skin depth also will change. This relationship between skin depth and temperature enables ferromagnetic material based instruments to achieve auto regulation.
The heating elements of surgical devices have been constructed from ferromagnetic material which is selected to have a Curie temperature at or near the auto-regulation temperature desired for a particular surgical application. As RF current passes through the ferromagnetic material, the heating element will resistively heat to approximately the Curie temperature. Once the cutting edge contacts tissue, both it and the area surrounding it will cool to a level below Curie temperature. In response to this Curie transition, skin depth will decrease which, in turn, results in an increased resistance of the cool region (the resistance being a function of the ferromagnetic material's resistivity multiplied by the length and divided by area). A corresponding increase in power supply will accompany this increase in resistance. The temperature then will tend to again increase due to resistive heating toward the Curie temperature. Thus, auto-regulation of the surgical component around the Curie temperature is achieved. See, for example, Eggers, U.S. Pat. No. 5,480,398, issued Jan. 2, 1996, entitled "Endoscopic Instrument with Disposable Auto-Regulating Heater"; and Eggers, et al., U.S. Pat. No. 5,480,397, issued Jan. 2, 1996, entitled "Surgical Instrument with Auto-Regulating Heater and Method of Using Same".
A disadvantage associated with resistively heated devices, including those which employ ferromagnetic heating elements, is concerned with a lack of sufficient localization of heat at the blunt cutting edge. In this regard, the entire heating element, including the support for its cutting edge, is heated toward a Curie temperature suited for cutting. This poses a risk that the support portion of the heating element may contact tissue or organs not selected for incision. Additionally, since a larger portion of the heating component is heated, the time period required for the cutting region to cool down to safe levels posing no threat of burn can be quite significant. This time period, for example, may be ten seconds or more, an interval, which in a surgical environment, is considered excessive, 2-4 seconds being considered acceptable, a starting condition interval to which surgeons are accustomed. Further, during laparoscopic or endoscopic procedures, the view of the surgeon is confined to a camera-generated two-dimensional image at a monitor, such as a TV screen. The heated element, however, may be moved out of the camera's limited field of view during the several seconds which are required for cool down, thereby posing a risk to tissue located adjacent the heated tip.
Another disadvantage associated with resistively heated devices has been concerned with the requirement that they must be powered by a specially designed or dedicated power supply. These dedicated electrical drive systems generally are configured to be unique to the properties of a particular heating element and are not of a universal nature, such that they would be usable with different surgical implements. In order to maximize the auto-regulation effect, the energy source used to apply power to the heating element preferably operates at a substantially constant current. Under constant current conditions, the amount of joulean power generated per unit length (i.e., current.sup.2 .times. resistance) depends only on the effective resistance of the heating element, R.sub.eff. The heating element resistance changes significantly as the temperature rises above the Curie temperature since the skin depth and associated current-conduction area, A, increases significantly above the Curie temperature. If a constant-voltage, alternating current power supply were used, the low heating element resistance which occurs when the heating element temperature exceeds the Curie temperature would result in a "thermal run-away" condition since joulean power dissipation, P, is inversely proportional to the resistance as shown below: ##EQU2## where P is joulean power dissipation, R.sub.eff is the effective resistance of the heating element, and V is constant voltage. For constant voltage V, as R.sub.eff decreases substantially above the Curie temperature, the power dissipation increases substantially resulting in excessively high heating element temperatures.
As a consequence of the foregoing considerations, practitioners have found it necessary to provide device dedicated electrosurgical generators for powering thermal implements. Of course, such added equipment requirements pose budgetary concerns to health care institutions.